Stephen king's greatest creation, Pennywise, is not actually a real clown. It is a malvolient entity that takes the shape of the thing you are most afraid of.
Humans cannot comprehend it's true form and anybody who has ever seen it has gone mad. Pennywise is actually a female, but preferes the shape of a male clown, because it attracts foolish children that eventually become the monster's food

1. Pennywise the Dancing Clown (Robert "Bob" Gray or It) is actually a pregnant female that originated in a void containing and surrounding the Universe. It's final form in the physical realm is a 15 foot high black spider, which is the closest the human mind can get to understanding It's actual physical form. It's natural form exists in a realm beyond the physical called the deadlights. The closest description we get in the book (IT by Stephen King) of this is "an endless hairy thing which was made of light and nothing else, orange light, dead light that mocked life."